Hyponatremia associated with an inappropriate AVP secretion characterizes the syndrome of inappropriate antidiuretic hormone secretion (SIADH); its diagnosis is often challenging, since its biochemical features overlap with those of the cerebral/renal salt wasting syndrome (C/RSW), a rare and controversial but severe hyponatremic condition found mainly in patients with cerebral tumors [11–16]. The gene discussed is AVP; the disease is Hyponatremia.